1. Field of the Invention
This invention relates to silicon-containing steel sheets having not only excellent magnetic properties but also good adhesion to a coating.
2. Related Art Statement
On the border of the energy crisis since several years, industry strongly tends to request electrical machinery and apparatus having less power loss. For this purpose, it becomes important to develop electromagnetic steel sheets having much lower iron loss as a core material for these machineries and apparatuses.
As the conventional method of producing grain oriented silicon steel sheets, there is usually performed a method wherein a starting material containing, for example, 2.0-4.0% by weight (hereinafter shown by % simply) of Si is hot rolled and subjected to a heavy cold rolling at once or two-times cold rolling through an intermediate annealing step to provide a final sheet thickness, and then the resulting cold rolled sheet was decarburization-annealed, coated with a slurry of an annealing separator composed mainly of MgO and wound in the form of a coil, and thereafter the coil is subjected to secondary recrystallization annealing and purification annealing (these two annealing steps are usually performed in one process. Hereinafter, the term "final annealing" is used) and further to a phosphate insulation coating if necessary.
In the above purification annealing, a forsterite (Mg.sub.2 SiO.sub.4) coating is formed by reacting an oxide layer of SiO.sub.2 produced on the surface of the steel sheet after the decarburization annealing with MgO contained in the annealing separator.
The grain oriented silicon steel sheets are obtained by aligning secondary recrystallized grains into (110)[001] orientation or Goss orientation through the above production steps and mainly used as a core for transformers and other electrical machineries. For this end, they are required to have a high magnetic flux density (exemplified by B.sub.10 value) and a low iron loss (exemplified by W.sub.17/50 Value) as the properties of the grain oriented silicon steel sheet. Particularly, it is recently demanded even more to reduce the iron loss for lessening the power loss of the transformer or the like from a viewpoint of energy-saving.
The iron loss of the silicon steel sheet is a sum of eddy current loss and hysteresis loss. As an effective means for reducing the iron loss of the silicon steel sheet, there is a method of reducing the sheet thickness, which mainly reduces the eddy current loss and largely contributes to the reduction of iron loss and hence the energy-saving. However, as the sheet thickness becomes not more than 11 mil, the ratio of the hysteresis loss occupied in total iron loss rapidly increases. As a factor effecting the hysteresis loss, mention may be made of orientation of crystal grain, amount of impurities, influence of surface coating, roughness of sheet surface and the like.
As a method of reducing the hysteresis loss by particularly improving the surface properties of the steel sheet, for instance, Japanese Patent Application Publication No. 52-24,499 proposes a method wherein a grain oriented silicon steel sheet after final annealing is pickled to remove oxides from the surface and is then rendered into a mirror state by subjecting it to a chemical polishing or an electrolytic polishing. Furthermore, Japanese Patent Application Publication No. 56-4,150 discloses a technique wherein the surface of the grain oriented silicon steel sheet is subjected to a chemical or electrolytic polishing after the removal of non-metallic substance and then coated with a ceramic thin film. And also, Japanese Patent laid open No. 60-89,589 discloses a technique wherein the surface of the grain oriented silicon steel sheet after the secondary recrystallization using an annealing separator composed mainly of alumina is subjected to a chemical or electrolytic polishing after the removal of oxides from the surface. Moreover, Japanese Patent laid open No. 60-39,123 discloses a technique wherein the grain oriented silicon steel sheet is subjected to a chemical or electrolytic polishing without direct pickling after the amount of oxide formed on the surface is controlled by using an annealing separator composed mainly of alumina.
However, these techniques clearly show the effect of reducing the iron loss, but they are not yet practised in industry. Because, in case of the chemical polishing, HF+H.sub.2 O.sub.2, H.sub.3 PO.sub.4 +H.sub.2 O.sub.2 or the like used as a polishing solution is expensive, resulting in an increase of the cost. On the other hand, in case of the electrolytic polishing, all of phosphoric acid bath, sulfuric acid bath, phosphoric acid-sulfuric acid bath, perchloric acid bath and the like have a high concentration of acid as a main ingredient and also contain a chromate, fluoric acid, organic compound or the like as an additive, so that they are high in cost and cause many unsolved problems on homogeniety, productivity, premature degradation of solution and the like when treating a great amount of steel sheet.
Furthermore, a great drawback obstructing the industriallization is that the insulation coating is hardly adhered onto the mirror finished surface of the sheet. That is, the conventionally known phosphate coating, ceramic coating and the like are poor in adhesion property due to the mirror surface and are not durable in practical use.